1. Field of the Invention
The present invention relates to a semiconductor device including a metal-insulator-semiconductor filed effect transistor (MISFET) and a method of manufacturing the same.
2. Related Background Art
It is known that in a MISFET or MOSFET, a hot carrier is generated as a result of electric field concentration at a gate edge, thereby to degrade the reliability of gate breakdown voltage. In order to prevent this, side portions of the gate are oxidized to thicken an insulating layer at the gate edge obtained by oxidizing the gate side portion, i.e., a reoxidized layer, to moderate the electric field intensity near the gate edge. However, a sufficient thickness of the reoxidized layer is required to appropriately moderate electric field. If a reoxidized layer 12 with a sufficient thickness is formed as shown in FIG. 8, this oxidized layer 12 may hinder subsequently-performed very-low-acceleration ion implantation or impurity doping using plasma, using gate electrodes 8a and 8b as masks, for forming an n-type extension layer 16 and a p-type extension layer 17 having a lower impurity concentration than n-type source/drain regions 20 and p-type source/drain regions 21. In FIG. 8, the reference numeral 1 denotes an n-type semiconductor substrate, 2a denotes a p-type semiconductor region, 2b denotes an n-type semiconductor region, 4 denotes a device isolating insulating layer, and 6a and 6b denote gate insulating layers.
Generally, polycrystalline silicon-germanium is used as a material of a gate electrode to activate an impurity (e.g., boron). When the reoxidized layer 12 with a sufficient thickness is formed as shown in FIG. 8, the edges of the gate electrodes 8a and 8b have a higher resistance value than the central portion 34 since deactivation of the impurity doped to make polycrystalline silicon-germanium conductive occurs at the side portions of the gate electrodes 8a and 8b. In a gate electrode which is particularly miniaturized, the proportion of the above-described deactivated portion in the gate electrode increases, thereby to form a depletion layer in the gate electrode. Accordingly, the capability of driving current of transistor is reduced, and the performance of MISFET is degraded.
Besides having a higher impurity activation ratio than polycrystalline silicon, which has conventionally been used as a material of gate electrode, polycrystalline silicon-germanium has a property that the band gap thereof is lower than that of polycrystalline silicon. In order to achieve a low threshold value with such a property, the substrate impurity concentration should be reduced as compare with the case where polycrystalline silicon is used. Generally, in order to inhibit the short-channel effect, the substrate impurity concentration should be set to be as high as possible. Accordingly, if polycrystalline silicon-germanium is used as the material of gate electrode, it is possible that the performance of the MISFET is degraded due to the short-channel effect. The above-described problem is especially noticeable in P-type MISFETs.